You don't need a degree to join. With a Police Constable Degree Apprenticeship (PCDA) you can earn and learn on the job, and you’ll end up with a degree in Professional Policing Practice at the end of the programme. Many forces are recruiting officers in this way right now - you can apply through your chosen force.
For some forces you can still apply via the traditional entry route – following the Initial Police Learning and Development Programme (IPLDP) and a two-year probation period.
Or if you want to get a degree before you join, some universities offer a specific pre-joining degree. If you wish to become a police officer through this route, you should search for a university offering the Professional Policing Degree. You can then apply to one or more forces as your degree is coming to an end.